This invention relates generally to storage tube devices and is particularly advantageous for scan converters used in applications requiring a high degree of fidelity in their data processing operation.
One application of the subject invention relates to scan converter devices wherein information is stored within a storage tube by a scanning electron beam that impresses a charge pattern on the dielectric of a target element; and the stored charge pattern is readout for extended periods of time by means of a read beam. The target structure of the scan converter tube may, for example, comprise coplanar conductive areas and dielectric areas. The conductive areas, which serve as target control elements, also act as the receptor for current during the read mode of operation; which current is amplified to provide the output data signal from the scan converter. The transfer function for the data readout operation of a typical scan converter tube is shown in FIG. 6. For most storage tubes, with a fixed value of cathode-to-target potential, there is only 4 or 5 volts range of dielectric charge between cutoff and saturation of the output current.
In applications requiring high fidelity of data conversion, a major problem restricting the use of storage tube devices is the presence of shading, i.e. nonuniform output for uniform or single valued writing on the target. Such shading is particularly undesirable in display type applications, since it results in all data being imprinted with the "map" of the nonuniform target characteristics. The shading or nonuniformities are caused by a variety of target characteristics and beam landing effects, and could be 20 percent or more. Also, it has been found that if the bias on the target's conductor is set at different levels the shading remains generally the same.
One aspect of the subject invention was recognition of the fact that the shading can be equated to the differential charging of the target during the write mode of operation. For example, for the scan converter tube whose readout transfer characteristics are shown in FIG. 6 and with a typical operating voltage of a +5 volts on the target's conductor, the range of charge stored on the dielectric would be 5 volts over the 5 to 95 percent range of the output current; and so if shading were 20 percent it could be equated to a 1 volt error in the stored charge pattern. Another aspect of the subject invention is recognition of the fact that the value of this shading error is relatively constant and if the target's dielectric were charged over a wider range, for example, 16 volts, the shading would be reduced; for example, to 1 volts out of 16 volts or 6 percent.
Charging the target's dielectric over a 16 volt range instead of the 5 volts normally used, only requires an increase in the intensity of the write beam current and/or an increase in the writing time; and for most storage tubes such an increase in writing range does not present any difficulties. However, readout of a 16 volt charge on the dielectric with a constant potential between the cathode and the target's conductor would result in a large portion of the written information being either in cutoff (the output below 5 percent of full output current) or a saturation (above 95 percent of full output current). A significant aspect of the subject invention is the recognition of the fact that the set of output current ranges for various target conductor-to-cathode voltages and dielectric charge levels, is such that if the target conductor-to-cathode voltage were properly adjusted during data readout a greatly increased range of stored charge could be handled without distortion.